The present invention relates to a base body of a reflecting mirror and a method for the preparation thereof. More particularly, the invention relates to a base body of a large-sized reflecting mirror used in reflecting astronomical telescopes and for collimation or diffusion of light beams, which is characterized by outstandingly light weight and still is free from any adverse influences on the dimensional precision and accuracy of the mirror surface due to mechanical deformation by the weight of the body per se and changes in the ambient temperature, as well as to a method for the preparation of such a base body of a reflecting mirror.
Reflecting mirrors in the prior art used in astronomical telescopes or for collimation or diffusion of light beams are prepared by lapping and polishing a surface of a mirror base made from fused quartz glass or high-silica glass to have a surface with flatness or a specified curvature of high precision, and providing the thus polished surface of the base body with a reflecting layer of a metal such as aluminum by the method of, for example, chemical vapor deposition at a temperature of 400.degree. to 800.degree. C. to give a reflecting surface. Such a reflecting mirror is used usually by being mounted on a supporting stand in a movable or rotatable fashion to facilitate taking a desired disposition, It is essential for the base body of a reflecting mirror that the base body has such physical properties that the accuracy of the mirror surface is not affected by various outer conditions such as changes in the ambient temperature to cause thermal expansion or shrinkage and changes in the disposition of the mirror to cause mechanical deformation of the base body by gravity.
When the reflecting mirror is relatively small, for example, having a diameter of 20 cm or smaller, the above mentioned requirements for the mirror base can be readily satisfied. Along with the recent trend that reflecting mirrors of larger and larger size, for example, having a diameter of 1 meter or even larger are demanded with an object to enhance the efficiency of the mirror system, the above mentioned requirements for the mirror base can be satisfied with increasing difficulties to ensure high accuracy of the reflecting surface. Namely, other than the high temperature at which the vapor deposition of the metal layer for the reflecting surface is performed, even a very slight change in the temperature of the mirror body caused by the changes in the ambient temperature and by the irradiation with high-energy light beams may cause a great thermal expansion of the base body so that the mirror surface is sometimes subject to warping or undulation resulting in a decrease in the performance of the reflecting mirror. This is the reason that the mirror base is formed from fused quartz glass or high-silica glass having an outstandingly small thermal expansion coefficient.
Besides the above mentioned thermal expansion or shrinkage, another serious problem in a large-sized reflecting mirror is the mechanical deformation of the mirror base, because a large-sized reflecting mirror naturally has a large weight, so that the mirror base is under a great influence of gravity to cause deformation of the mirror base in different ways as the disposition of the mirror is changed by being rotated or moved on the supporting stand. Accordingly, various attempts and proposals have been made in the prior art for decreasing the body weight of a reflecting mirror by the improvement of the structure of the base body of the mirror without sacrifice in the mechanical strength as a support of the reflecting surface, to comply with the practical requirement to ensure good operability of a large-sized reflecting mirror having a glass-made mirror base.
For example, Japanese Patent Publication 63-57761 discloses a light-weight glass-made base body of a reflecting mirror for astronomical telescopes, which consists of a front plate, i.e. the surface plate for forming the reflecting surface by metal plating thereon, a rear plate or backing plate as a base for supporting the front plate and a latticework therebetween composed of a plural number of rows of pipes made from fused quartz glass. In the latticework of pipes, each pipe of the pipe rows is contacted in a cross-stitch arrangement with the two pipes in the respective adjacent rows forming contacting lines or contacting zones while the wall thickness of the pipes is smaller along the above mentioned contacting lines or zones than in the other portions of the pipe walls and the pipes are joined together into an integral latticework by welding along the contacting lines or zones. Such a complicated latticework structure of the intermediate layer between the front plate and the rear plate of the base body, however, is industrially very disadvantageous because of the very large costs for the preparation thereof. In addition, the mirror base having such a latticework structure has poor mechanical strength in the direction within the surface plane so as not to withstand the high-precision lapping and polishing works of the optical surface, before plating with a metal, to have a desired flatness or curvature of the reflecting surface.
Moreover, it is a very difficult matter to obtain the pipe elements forming the latticework having an exactly equal effective height so that the front plate after polishing supported by the latticework unavoidably retains a strain corresponding to the height difference in the pipe elements forming the latticework to cause deformation or undulation of the reflecting surface after lapse of a certain length of time. The rigidity or such a latticework is of course inherently anisotropic and differs between the directions perpendicular to and parallel with the reflecting surface, so that the reflecting mirror having such a base body can hardly be used when the mirror must take different dispositions by being rotated or moved on the supporting stand due to the poor accuracy of the reflecting surface when the disposition of the mirror is varied.
Further, Japanese Patent Publication 61-26041 discloses another light-weight glass-made base body of a reflecting mirror for astronomical telescopes. The base body of fused quartz glass also consists of a front plate, a rear plate and an interposed latticework layer therebetween integrated into a body by welding. The latticework is prepared by putting plate-formed and/or tubular lattice elements on a supporting plate to form a lattice and filling the spaces formed between or surrounded by the lattice elements with tiny pieces of the same glass susceptible to sintering, followed by sintering of this assemblage as fastened with a graphite ring in a furnace under a non-oxidizing atmosphere. The thus prepared latticework is sandwiched between the front plate and the rear plate and welded together into an integral base body to be finished by polishing the surface of the front plate. Such a base body of a reflecting mirror is industrially disadvantageous and not practical due to the very lengthy and troublesome procedure of manufacture, with consequently very high costs, in addition to the problem that the front plate bonded to the latticework by welding retains substantial strains at the welded portions to greatly affect the dimensional accuracy of the reflecting surface.